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 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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package java.util.stream;

import java.util.Spliterator;
import java.util.function.IntFunction;

/**
 * Helper class for executing <a href="package-summary.html#StreamOps">
 * stream pipelines</a>, capturing all of the information about a stream
 * pipeline (output shape, intermediate operations, stream flags, parallelism,
 * etc) in one place.
 *
 * <p>
 * A {@code PipelineHelper} describes the initial segment of a stream pipeline,
 * including its source, intermediate operations, and may additionally
 * incorporate information about the terminal (or stateful) operation which
 * follows the last intermediate operation described by this
 * {@code PipelineHelper}. The {@code PipelineHelper} is passed to the
 * {@link TerminalOp#evaluateParallel(PipelineHelper, java.util.Spliterator)},
 * {@link TerminalOp#evaluateSequential(PipelineHelper, java.util.Spliterator)},
 * and {@link AbstractPipeline#opEvaluateParallel(PipelineHelper, java.util.Spliterator,
 * java.util.function.IntFunction)}, methods, which can use the
 * {@code PipelineHelper} to access information about the pipeline such as
 * head shape, stream flags, and size, and use the helper methods
 * such as {@link #wrapAndCopyInto(Sink, Spliterator)},
 * {@link #copyInto(Sink, Spliterator)}, and {@link #wrapSink(Sink)} to execute
 * pipeline operations.
 *
 * @param <P_OUT> type of output elements from the pipeline
 * @since 1.8
 */
abstract class PipelineHelper<P_OUT> {

  /**
   * Gets the stream shape for the source of the pipeline segment.
   *
   * @return the stream shape for the source of the pipeline segment.
   */
  abstract StreamShape getSourceShape();

  /**
   * Gets the combined stream and operation flags for the output of the described
   * pipeline.  This will incorporate stream flags from the stream source, all
   * the intermediate operations and the terminal operation.
   *
   * @return the combined stream and operation flags
   * @see StreamOpFlag
   */
  abstract int getStreamAndOpFlags();

  /**
   * Returns the exact output size of the portion of the output resulting from
   * applying the pipeline stages described by this {@code PipelineHelper} to
   * the the portion of the input described by the provided
   * {@code Spliterator}, if known.  If not known or known infinite, will
   * return {@code -1}.
   *
   * @param spliterator the spliterator describing the relevant portion of the source data
   * @return the exact size if known, or -1 if infinite or unknown
   * @apiNote The exact output size is known if the {@code Spliterator} has the {@code SIZED}
   * characteristic, and the operation flags {@link StreamOpFlag#SIZED} is known on the combined
   * stream and operation flags.
   */
  abstract <P_IN> long exactOutputSizeIfKnown(Spliterator<P_IN> spliterator);

  /**
   * Applies the pipeline stages described by this {@code PipelineHelper} to
   * the provided {@code Spliterator} and send the results to the provided
   * {@code Sink}.
   *
   * @param sink the {@code Sink} to receive the results
   * @param spliterator the spliterator describing the source input to process
   * @implSpec The implementation behaves as if:
   * <pre>{@code
   *     intoWrapped(wrapSink(sink), spliterator);
   * }</pre>
   */
  abstract <P_IN, S extends Sink<P_OUT>> S wrapAndCopyInto(S sink, Spliterator<P_IN> spliterator);

  /**
   * Pushes elements obtained from the {@code Spliterator} into the provided
   * {@code Sink}.  If the stream pipeline is known to have short-circuiting
   * stages in it (see {@link StreamOpFlag#SHORT_CIRCUIT}), the
   * {@link Sink#cancellationRequested()} is checked after each
   * element, stopping if cancellation is requested.
   *
   * @param wrappedSink the destination {@code Sink}
   * @param spliterator the source {@code Spliterator}
   * @implSpec This method conforms to the {@code Sink} protocol of calling {@code Sink.begin}
   * before pushing elements, via {@code Sink.accept}, and calling {@code Sink.end} after all
   * elements have been pushed.
   */
  abstract <P_IN> void copyInto(Sink<P_IN> wrappedSink, Spliterator<P_IN> spliterator);

  /**
   * Pushes elements obtained from the {@code Spliterator} into the provided
   * {@code Sink}, checking {@link Sink#cancellationRequested()} after each
   * element, and stopping if cancellation is requested.
   *
   * @param wrappedSink the destination {@code Sink}
   * @param spliterator the source {@code Spliterator}
   * @implSpec This method conforms to the {@code Sink} protocol of calling {@code Sink.begin}
   * before pushing elements, via {@code Sink.accept}, and calling {@code Sink.end} after all
   * elements have been pushed or if cancellation is requested.
   */
  abstract <P_IN> void copyIntoWithCancel(Sink<P_IN> wrappedSink, Spliterator<P_IN> spliterator);

  /**
   * Takes a {@code Sink} that accepts elements of the output type of the
   * {@code PipelineHelper}, and wrap it with a {@code Sink} that accepts
   * elements of the input type and implements all the intermediate operations
   * described by this {@code PipelineHelper}, delivering the result into the
   * provided {@code Sink}.
   *
   * @param sink the {@code Sink} to receive the results
   * @return a {@code Sink} that implements the pipeline stages and sends results to the provided
   * {@code Sink}
   */
  abstract <P_IN> Sink<P_IN> wrapSink(Sink<P_OUT> sink);

  /**
   *
   * @param spliterator
   * @param <P_IN>
   * @return
   */
  abstract <P_IN> Spliterator<P_OUT> wrapSpliterator(Spliterator<P_IN> spliterator);

  /**
   * Constructs a @{link Node.Builder} compatible with the output shape of
   * this {@code PipelineHelper}.
   *
   * @param exactSizeIfKnown if >=0 then a builder will be created that has a fixed capacity of
   * exactly sizeIfKnown elements; if < 0 then the builder has variable capacity.  A fixed capacity
   * builder will fail if an element is added after the builder has reached capacity.
   * @param generator a factory function for array instances
   * @return a {@code Node.Builder} compatible with the output shape of this {@code PipelineHelper}
   */
  abstract Node.Builder<P_OUT> makeNodeBuilder(long exactSizeIfKnown,
      IntFunction<P_OUT[]> generator);

  /**
   * Collects all output elements resulting from applying the pipeline stages
   * to the source {@code Spliterator} into a {@code Node}.
   *
   * @param spliterator the source {@code Spliterator}
   * @param flatten if true and the pipeline is a parallel pipeline then the {@code Node} returned
   * will contain no children, otherwise the {@code Node} may represent the root in a tree that
   * reflects the shape of the computation tree.
   * @param generator a factory function for array instances
   * @return the {@code Node} containing all output elements
   * @implNote If the pipeline has no intermediate operations and the source is backed by a {@code
   * Node} then that {@code Node} will be returned (or flattened and then returned). This reduces
   * copying for a pipeline consisting of a stateful operation followed by a terminal operation that
   * returns an array, such as:
   * <pre>{@code
   *     stream.sorted().toArray();
   * }</pre>
   */
  abstract <P_IN> Node<P_OUT> evaluate(Spliterator<P_IN> spliterator,
      boolean flatten,
      IntFunction<P_OUT[]> generator);
}
